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Y2 LCRS2 Neuroscience and Mental Health > Somatosensory System > Flashcards

Somatosensory System Flashcards

1
Q

What is a somatosensory modality?

A

A type of stimulus e.g. hot/cold/touch

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2
Q

List some differnet modalities and their respective receptors.

A

Touch, pressure, vibration, proprioception –> mechanoreceptor

Temp –> thermoreceptor

Nociception/pain –> nociceptor

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3
Q

What are A-alpha sensory fibres invovled in?

A

Proprioception

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4
Q

List the different types of sensory fibres and their function. What do these make up together?

A
  • A-alpha - proprioception of skeletal muscle
  • Aβ-fibres: innocuous mechanical stimulation
  • Aδ-fibres: noxious mechanical and thermal stimulation
  • C-fibers: noxious mechanical, thermal and chemical stimulation

Together they form a peripheral sensory nerve.

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5
Q

List the sensory fibres in order of fastest to slowest.

A 
A-beta = large, myeinated
A-delta = medium, myelinated
C = narrow, unmyelinayed so slowest
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6
Q

Which sensory receptors have free nerve endings?

A
  • Free nerve endings: thermoreceptors and nociceptors
  • Enclosed nerve endings: mechanoreceptors
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7
Q

What is the purpose of sensory receptors?

A

They are trasducers which convert energy from the environment into neuronal action potentials.

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8
Q

Which sensory fibres are thermoceptors made of? What type of receptors are they?

A
  • A-delta and C-fibres
  • NB: free nerve endings
  • Transient receptor potential ion channels (TRP)
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9
Q

Name the 4 heat activated and the 2 cold activated thermoceptors.

A

4 heat activated:

  • TRPV1-4

2 cold activated

  • TRPM8
  • TRPA1

NB: they are all activated at different temperatures.

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10
Q

Name the 4 different types of mechanoceptors. What is each of their functions?

A

Meissner’s corpuscle - fine discriminative touch, low frequency vibration

Merkel cells - light touch and superficial pressure

Pacinian corpuscle - deep pressure, high frequency vibration and tickling

Ruffini endings - continuous pressure, touch and stretch

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11
Q

Which mechanoceptor senses low frequency vibration? Which senses high frequency vibration?

A

Meissener’s corpuscle - low

Pacinican corpuscle - high

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12
Q

Which mechanoceptors are for sensing light touch and superficial pressure?

A

Merkel cells

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13
Q

What is an absolute threshold?

A

Point of intensity at which the person can JUST detect the presence of a simulus 50% OF THE TIME (absolute threshold)

Stimulus –> generator potesntial which when strong enough –> action potential to the spinal cord.

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14
Q

How do you generate a greater intensity stimulus? Describe the pathway.

A

Longer and larger stimulus –> greater generator potential –> more action potentials –> greater neurotransmitter release–> greater intensity

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15
Q

What are tonic receptors? What is special about them?

A
  • Detect continuous stimulus strength
  • They DO NOT adapt or adapt very slowly -
  • Action potentials CONSTANTLY being sent to keep brain informed of the status of the body as long as the stimulus is present
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16
Q

Give an example of a tonic receptor.

A

Merkel cells - slowlsy adapting for superficial pressure and fine touch to be perceived.

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17
Q

What are phasic receptors?

A
  • Receptors which adapt very quickly to a stimulus
  • Detect change in stimulus strength
  • Transmit impulse at START and END of stimulus
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18
Q

Give an example of a phasic receptor.

A

Pacinian receptor - sudden pressure excites the receptor and another signal is transmitted when pressure is released.

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19
Q

What is a receptive field on the skin?

A

A region which causes activation of a single sensory neuron when activated.

Different areas of the body have differnet sizes of receptive fields

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20
Q

Describe how the size of a receptive fields affects sensation.

A

Fingers have densely packed mechanoceptors with small receptive fields while the back has single primary sensory fibres over larger areas.

Large receptive field –> cell can detect changes over a wider area but less precise perception

Small receptive fields –> detection of fine detail over a small area = precise perception

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21
Q

What is a dermatome?

A

An area of the skin supplied by a specfic spinal nerve = somatotopic organisation of the nervous system

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22
Q

Define two point discrimination. What is this related to?

A
  • Minimum distance at which two points are perceived as separate
  • Related to the size of the receptive field
  • E.g. on the hand the receptor fields are smaller so you would probably sense two points a certains distance apart. However, the points would be perceived as one on the back because the receptive fields are larger.*
  • Two points would have to be 40mm apart on the abck compared to 10mm on the hand.*
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23
Q

Where are the cell bodies of sensory neurons located?

A
  • In the body: dorsal root ganglia
  • In the face: trigeminal ganglia
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24
Q

What are the two types of dorsal horn neurons?

A
  1. Projection neurons - those that have axons which project to the brain
  2. Interneurons - those that have axons which remain in the spinal cord
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25
Q

Which neuorns are responsible for preventing overlap between receptive fields and what is this called?

A

Interneurons within the dorsal horn of the spinal cord

This is called lateral inhibition.

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26
Q

Describe the concept of lateral inhibition.

A
  • Receptive fields can overlap so it is difficult to distinguish between 2 stimulus locations
  • Lateral inhibition prevents overlap and facilitates accuracy in localising a stimulus –> enhanced sensory perception/DISCRIMINATION
  • This is mediated by inhibitory interneurons in the dorsal horn of the spinal cord
27
Q

What sensory information is sent to the superficial layers of the dorsal horn? What is sent to the deep dorsal horn?

A
  • Innocuous mechanical stimuli (fine touch and vibration)- deep
  • Pain and temperature - superficial
28
Q

Which sensory pathways are involved in detecting fine discriminate touch and vibration?

A

Dorsal column pathways - detect innocuous mechanical stimuli

29
Q

What are the sensory fibres of the dorsal column pathways? Where do they enter the spinal cord?

A

A-beta sensory fibres - enter via the dorsal horn and enter the ascending dorsal column pathways

30
Q

Which tract in the dorsal column pathway carries sensory information from:

a) the lower limb
b) the upper limb

A

a) Gracile tract - information conveyed from lower limbs and body (below T6) travels ipsilaterally along the gracile tract
b) Cuneate tract -information conveyed from upper limbs and body (above T6) travels ipsilaterally along the cuneate tract

31
Q

Where do sensory fibres from the gracile and cuneate tract first synapse?

A
  • Fibres from gracile tract first synapse in the GRACILE NUCLEUS in the medulla
  • Fibres from the cuneate tract first synapse in the CUNEATE NUCLEUS in the medulla
32
Q

Describe the pathways of 1st , 2nd and 3rd order sensory neurons of the dorsal column pathway.

A
  • A-beta fibres–> dorsal horn of spinal cord
    • from lower limbs –> gracile tract
    • from upper limbs –> cuneate tract
  • 1st order neurons then terminate in the medulla
    • Gracile tract fibres in gracile nucleus
    • Cuneate tract fibres in cuneate nucleus
  • Second order neurons decussate in the caudal medulla forming the contralateral medial lemniscus tract
  • .. and terminate in ventral posterior lateral nucleus of thalamus
  • Third order neurons from VPL project to the somatosensory cortex
33
Q

What is the contralateral medial lemniscus tract and where does it travel?

A

Dorsal column sensory tract which travels from medulla to thalamus

34
Q

Describe the topographic representation of the body in the VPL

A

Lower extremities are more lateral

35
Q

What is shown by the somatosensory humunculus?

A

Size of somatotopic areas is proportional to density of sensory receptors in that body region

36
Q

Which ascending tract carries sensory information about pain, temperature and crude touch? What are its two pathways?

A

Spinothalamic (anterolateral) pathway

  • Lateral pathway - Pain and temperature
  • Anterior pathway - Crude touch ascends
37
Q

Describe the pathways of 1st, 2nd and 3rd order sensory neurons in the spinothalamic tract.

A
  • 1st order/primary afferent neurons terminate as soon as they enter the spinal cord
  • 2nd order neurons decussate immediately in the spinal cord and form the spinothalamic tract
  • 2nd order neurons terminate in VPL of thalamus
  • 3rd order neurons project to the somatosensory cortex - pain and temperature localisation is not as precise as touch localisation
38
Q

What are the key differences between dorsal column and spinothalamic tracts?

A
  • Dorsal column is for transmitting sensory info about light touch, vibration and discriminative touch. Decussate in brainstem.
  • Spinothalamic pathway is for transmitting sensory info about pain, temperature and coarse touch. Decussate in spinal cord.
  • Similarities:*
  • Second order neurons terminate in the VPL*
  • There is a topographic representation in the VPL (lower extremities more lateral in both pathways)*
39
Q

What type of testing is used to assess the integrity of ascending pathways clinically?

A

Qualitative sensory testing - different tools can be used to assess different sensory modalities e.g. tuning fork, brush, needle.

40
Q

What would this person be able to feel when assessed using QST? When would you get this type of damage to the spinal cord?

A
  • Spinothalamic tracts are located anteriorly so loss of pain and temperature perception bilaterally below the level of the lesion
  • Dorsal columns are intact so innoculous mechanical stimulation, 2 point discrimination and vibration perception retained

Common when there is blockage of an anterior spinal artery

41
Q

What are the consequences of anterior spinal artery blockage?

A

Ischaemic damage to the anterior part of the spinal cord

  • Loss of pain and temp. perception below the level of the lesion - affected spinothalamic tracts
  • Retained touch, vibration and 2 point discrimination - intact dorsal columns
42
Q

What is the IASP definition of pain?

A

An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.

43
Q

Which sensory fibres are associated with nociceptors? What signals do they mediate?

A

A-delta - mediate sharp, intense or first pain

C-fibres - mediate dull aching or second pain

44
Q

What are the two types of A-delta fibres found in nociceptors and how do they differ?

A

Type 1: transmit noxious mechanical stimuli

Type 2: noxious heat

45
Q

Describe the features of C-fibres found in nociceptors. What types of stimuli do they transmit?

A
  • C-fibres mediate dull, aching or second pain
  • They are POLYMODAL - transmit noxious, mechanical and chemical stimuli
  • Unmyelinated so slower
46
Q

Which neurotransmitter is involved in nociceptive processing in the spinal cord?

A

Glutamate (action on superficial dorsal horns)

47
Q

What are the two main components of pain? Which tracts mediate these components?

A

Sensory and emotional aspects

Spinoreticular tract - emotional component (spinal cord–>parabrachial area in brainstem–>limbic system)

Lateral spinothalamic - sensory component

48
Q

Which parts of the brain light up on an fMRI in response to pain?

A
  • Cortex:
    • SI
    • SII
    • Insula cortex
    • Anterior cingulate cortex
    • Prefrontal cortex
  • Amygdala
  • Cerebellum
  • Brainstem
49
Q

Describe the Gate Control Theory.

A

A theory that pain could be inhibited in the spinal cord before it reaches the brain.

We now know that activation of A-beta fibres can inhibit pain signals in the dorsal horn by activating inhibitory interneurons which block the signal transmitted by A-delta or C-fibres so…

If you hurt yourself and rub your knee to “make it better” there is actually some sense in that

50
Q

How are descending pathways involved in pain control?

A

Descending noradrenergic pathways from the LC in brainstem can inhibit nociceptive signalling in the dorsal horn

Descending serotonergic pathways from the RVM in medulla can facilitate nociceptive signalling in the dorsal horn

Clinical context: descending inhibition is probably lost in chronic pain patients –> somatosensory disruption.

51
Q

Where is the primary control center for descending pain modulation?

A

PAG - periaqueductal grey

52
Q

Which monoamines are invovled in facilitation and inhibition of nociceptive processing in the dorsal horn?

A

Serotonin and noradrenaline respectively.

53
Q

What is neuropathic pain?

A

A lesion or disease of the somatosensory system

E.g. sciatica, diabetic, chemotherapy, post-surgical

54
Q

What is nociceptive pain?

A

Noxious stimulation of a nociceptor (somatic or visceral)

E.g. affecting muscles, skin, ligamnets, bones, viscera.

55
Q

What is the difference between allodynia and hyperalgesia?

A

Allodynia is pain due to a stimulus that does not normally provoke pain

Hyperalgesia is increased pain from a stimulus t_hat normally provokes pain._ It can be primary or secondary.

56
Q

Describe how peripheral sensitisation causes primary hyperalgesia.

A
  1. Inflammatory changes occur at nerve endings of C-fibres
  2. Inflammatory mediators e.g. bradykinin, histamine etc bind to receptors –> C-fibres change
  3. C-fibres then have a lower activation threshold for painful stimuli
  4. This manifests as primary hyperalgesia
57
Q

Describe how central sensitisation can cause secondary hyperalgesia.

A
  1. During chronic pain, changes occur at the spinal cord
  2. On-going activation of C-fibres –> NMDA receptor-mediated synaptic plasticity
  3. This causes changes in central processing of adjacent A-delta fibres
  4. This causes sensitivity to spread away from the site of injury on the skin
  5. –> reduced threshold to peripheral stimuli at an adjacent site to injury –>expansion or receptive field –> spontaneous pain
  6. –> secondary hyperalgesia
58
Q

How is neuropathic pain assessed?

A
  • Questionnaire
  • Sensory testing - brush, pin-prick, thermal stimulation
59
Q

Name the three neuropathic pain “clusters”/phenotypes.

A
  • Sensory loss
  • Thermal hyperalgesia
  • Mechanical hyperalgesia
60
Q

Compare the efficacy of different antidepressants in neuropathic pain.

A

SSRIs are not very effective, NNT(no. needed to treat) is about 7 (every 7th patient will receive about 50% pain relief)

SNRIs and TCAs are more efficacious in neuropathic pain

61
Q

Describe what happens to descending control in chronic pain.

A
  • There is loss of control of descending noradrenergic pathways which are protective
  • There is a shift towards descending facilitation (serotonergic pathway) in chronic pain patients
62
Q

Name a SNRI which has been used in chronic pain. Describe its mechanism.

A
  • Duloxetine
  • Enhances action of noradrenaline in the spinal cord
  • Works best in those who have deficient descending inhibition .
63
Q

What is neruomodulation and how can it be used in chronic pain patients?

A
  • Neuromodulation - non-invasive motor cortex stimulation (tDCS). A way of reducing chronic pain symptoms without using drugs.
  • Activates endogenous analgesic systems in the brain: PAG and anterior cingulate cortex.
  • Have been found to be effective in some patients but analegesic mechanisms are unclear

Y2 LCRS2 Neuroscience and Mental Health (17 decks)

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